CDMA mobile communication system and communication method

ABSTRACT

A radio communication system having a base station and a plurality of radio terminals, wherein each radio terminal having a transmission request transmits a reservation packet at arbitrary timing through a reservation channel in accordance with a CDMA scheme, and the base station assigns a traffic channel and a time slot to be used to each radio terminal requesting a reservation through a reply packet outputted onto a reply channel. On the reservation channel, a short spreading code corresponding to a matched filter is applied.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a mobile communication system and a communication method, and more particularly, to a reservation based mobile communication system, mobile terminal equipment, and communication method to which code division multiple access (CDMA) is applied.

[0003] 2. Description of the Related Art

[0004] Conventionally, a mobile communication system which employs a reservation based access control in a frequency division multiple access (FDMA) scheme is known, for example, as described in IEEE Transactions on Communications, Packet Switching in Radio Channels: “Part3-Polling and (Dynamic) Split-Channel Reservation Multiple Access”, COM-24, 8, (1976), pp. 832-845 (hereinafter called “prior art publication 1”).

[0005] In the reservation based access control, each of mobile terminals having a request for data transmission reserves a traffic channel to a base station through a reservation packet. The base station, after scheduling traffic channels and transmission timing (time slots) to be assigned to these mobile terminals, notifies each of the mobile terminals of transmission timing to be used on an assigned traffic channel through a reply packet. According to this reservation based access control, collision of packets on the traffic channel can be basically avoided.

[0006] As another example of reservation based control type communication system, for example, JP-A-6-311160, corresponding to U.S. patent application Ser. No. 08/230773 (hereinafter called “prior art publication 2”) has proposed such a communication system based on a time division multiple access scheme.

[0007] However, in the mobile communication systems in which the reservation based access control is applied to FDMA and TDMA schemes, as proposed by prior art publications 1 and 2, since respective mobile terminals send reservation packets through a reservation channel asynchronously with each other, a plurality of reservation packets can collide with a high possibility. Thus, repetitive retransmission of reservation packets obliged by the collision of packets constitutes a main cause of degrading the throughput of the entire communication system.

[0008] Meanwhile, as a standard for FPLMTS (Future Public Land Mobile Telecommunication Systems), the adoption of the code division multiple access scheme is regarded as promising. A CDMA mobile communication system has been proposed, for example, in JP-A-7-38496 corresponding to U.S. patent application Ser. No. 08/375679 (hereinafter called “prior art publication 3”). However, prior art publication 3 does not provide any useful information for solving the problem of a degraded throughput in the reservation based access control.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a mobile communication system and a communication method which employ a reservation based access control to realize a high throughput.

[0010] It is another object of the present invention to provide CDMA mobile terminal equipment and base station which solve the problem of collision of reservation packets to realize a high throughput.

[0011] To achieve the above objects, in a mobile communication system of the present invention, radio channels include a plurality of traffic channels used for transmitting upward data packets directed from mobile terminals to a base station and for transmitting downward data packets directed from the base station to the mobile terminals, a reservation channel used for transmitting reservation packets each indicative of a traffic channel assignment request from a mobile terminal to the base station, and a reply channel used for transmitting reply packets each indicative of a traffic channel through which data is transmitted and received from the base station to a mobile terminal, wherein the reservation, reply and traffic channels are applied with spread-spectrum in accordance with a CDMA scheme. The mobile communication system is characterized in that a mobile terminal having a request for data transmission transmits a reservation packet onto the reservation channel at arbitrary timing, the base station specifies a traffic channel and a time slot to be used by the requesting mobile terminal by a reply packet transmitted through the reply channel, and each mobile terminal transmits and receives a data packet in the time slot on the traffic channel, both specified by the reply packet.

[0012] Describing in greater detail, each of the reservation, reply and traffic channels is assigned a unique spreading code, for example, pseudonoise (PN). Particularly, the reservation channel is assigned a spreading code shorter than those assigned to other reply and traffic channels. The base station relies on a matched filter to identify a plurality of reservation packet signals having time-overlapped portions, transmitted from a plurality of mobile terminals, and to perform a receiving process on bit signals corresponding to each packet.

[0013] According to a preferred embodiment of the present invention, the base station, upon receiving a reservation packet from a mobile terminal, assigns a time slot on a traffic channel in accordance with a schedule control, and notifies each mobile terminal of the assignment result through a reply packet.

[0014] Also, for regulating a total number of simultaneously communicated packets, the base station periodically transmits a busy tone signal indicative of a traffic situation, such that each mobile terminal having a request for data transmission performs a reservation packet transmission control in accordance with the busy tone signal. Alternatively, the radio channels may be provided with a plurality of reply channels so as to specify a reply channel for each mobile terminal to receive the busy tone signal therethrough.

[0015] According to the present invention, time slots are defined in the traffic channels such that each mobile terminal transmits and receives data in a particular time slot specified by the base station. The reservation channel, on the other hand, is not provided with time slots, so that each mobile terminal having a request for data transmission transmits a reservation packet at arbitrary timing, thus facilitating the operation of transmitting the reservation packet in each mobile terminal.

[0016] Also, each mobile terminal performs a spectrum spreading or multiplies the reservation packet by a spreading code to generate a spread-spectrum reservation packet, where the spreading code has a period shorter than that applied to a data packet transmitted through a traffic channel, while the base station receives reservation packets using a matched filter.

[0017] In this case, even if two or more spread-spectrum control packets, modulated by the same spreading code, are partially overlapped on the time axis, the matched filter can identify received packets, provided that there is a timing deviation over one chip or more on the spreading code between the respective packets. Therefore, even if a plurality of mobile terminals generate reservation packets individually at arbitrary timing, a reception disabled condition caused by collision of these packets will occur with an extremely low possibility.

[0018] The foregoing and other objects, advantages, manner of operation and novel features of the present invention will be understood from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 illustrates an exemplary configuration of a mobile communication network to which the present invention is applied;

[0020]FIG. 2A is a diagram for explaining a protocol for a call set up process in a radio communication system according to the present invention;

[0021]FIG. 2B is a diagram for explaining a protocol for information transmission in the radio communication system according to the present invention;

[0022]FIG. 3 is a diagram for explaining a channel access control in a conventional radio communication system;

[0023]FIG. 4 is a diagram for explaining a channel access control in a radio communication system according to the present invention applying a CDMA scheme;

[0024]FIG. 5A illustrates a format for a reservation packet;

[0025]FIG. 5B illustrates a format for a reply packet;

[0026]FIG. 5C illustrates a format for an information transmission packet;

[0027]FIG. 6 is a block diagram illustrating the configuration of a base station;

[0028]FIG. 7 is a block diagram illustrating the configuration of a CDMA transceiver 50 in the base station;

[0029]FIG. 8A is a block diagram illustrating the configuration of a matched filter 70;

[0030]FIG. 8B is a diagram for explaining how the matched filter processes received reservation packets;

[0031]FIG. 9 is a block diagram illustrating the configuration of a packet separation circuit 80;

[0032]FIG. 10 is a block diagram illustrating the configuration of a packet controller 90 in the base station;

[0033]FIG. 11 is a block diagram illustrating the configuration of a mobile terminal;

[0034]FIG. 12 is a block diagram illustrating the configuration of a CDMA transceiver 110 in the mobile terminal;

[0035]FIG. 13 is a block diagram illustrating the configuration of a packet controller 130 in the mobile terminal; and

[0036]FIGS. 14A and 14B are diagrams for explaining a busy tone control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037]FIG. 1 illustrates an exemplary configuration of a mobile communication network to which the present invention is applied.

[0038] The illustrated mobile communication network comprises a public network 1 accommodating stationary terminals such as a telephone 3 or the like; and a mobile communication network 2 connected to the public network 1 and accommodating a plurality of base stations 4 (4 a, 4 b, . . .), wherein each base station 4 communicates with mobile terminals (radio terminals) 5 (5 a, 5 b, . . .) located in its service area (cell) through radio channels 6. On the radio channel, a CDMA packet transmission is applied because of its suitability to communications of multi-media information in which data, sound and image signals are mixed.

[0039]FIG. 2A shows a protocol for a call set up process in the radio communication system according to the present invention.

[0040] The call set up process includes two different sequences of operations: one is a sequence of operations for initially allocating local ID's (local addresses) to mobile terminals in a service area, and the other is a sequence of operations for allocating a link number to each mobile terminal for communicating with another destination terminal. The local ID is an address number having a reduced length than that of a unique address previously assigned to each mobile terminal. The use of this local ID results in reducing the length of a packet. The link number also has a similar effect to the local ID.

[0041] A procedure of the call set up process is common to the above-mentioned sequences of operations for allocating the local ID's and for allocating the link numbers. Specifically, the procedure comprises the steps of transmitting a control packet (reservation packet) 10 a for call set up from a terminal to a base station through a reservation channel 7; transmitting a control packet (reply packet) 11 a from the base station to the terminal through a reply channel 8; and transmitting a call set up data packet 12 a from the base station to the terminal through a traffic channel.

[0042] Address information indicative of a source is set in the control packet 10 a. Also, the address of a terminal required to receive the data packet 12 a and a time slot on the traffic channel 9 in which the data packet 12 a is to be received, are specified by the control packet 11 a, such that the terminal specified by this control packet 11 a receives the call set up data packet 12 a including location registration information (local ID number) or link information (link number) transmitted by the base station in the specified time slot on the traffic channel 9.

[0043] It should be noted that if the control packet 11 a has a sufficient length, the location registration information or the link information may be transmitted through the control packet 11 a, instead of utilizing the call set up data packet 12 a.

[0044] The reservation channel 7, reply channel 8, and traffic channels 9 are distinguished by PN codes which are applied to spread-spectrum. A plurality of traffic channels 9 can be formed by providing a plurality of PN codes for transmitting data packets.

[0045] The base station is provided, for example, with a management table for indicating a slot using situation on each traffic channel such that the base station schedules a slot for transmitting the data packet 12 a so as to minimize a waiting time of the terminal by referring to this management table.

[0046]FIG. 2B shows a protocol for transmitting user information (hereinafter simply called the “data”).

[0047] A terminal (transmitting terminal) having a request for data transmission utilizes a PN code for the reservation channel 7 to transmit a control packet (reservation packet) 10 b for requesting the assignment of a slot in which a data packet is to be transmitted. The base station, in response to this request, utilizes a PN code for the response channel 8 to transmit a control packet (reply packet) 8 b to the request transmitting terminal, thereby specifying a traffic channel 9 i and a time slot to be used by the request transmitting terminal. The request transmitting terminal, upon receiving the reply packet 11 b, sends the data packet 12 b at the timing of a specified time slot on the traffic channel 9 i.

[0048] The data packet 12 b is once received by the base station. The base station confirms a destination address of the data packet, and utilizes the PN code for the reply channel 8 to transmit a control packet 13 for specifying a destination terminal (receiving terminal) as well as a traffic channel 9 j and a time slot with which the receiving terminal is to receive the data packer 12 b, when the receiving terminal is a mobile terminal located in the service area of the base station. Then, the base station sends the received data packet 12 b from the request transmitting terminal as a data packet 14 in the specified time slot. The receiving terminal receives the data packet 14 transferred from the base station in the specified time slot on the traffic channel 9 j specified by the control packet 13.

[0049] According to the information transmission protocol described above, while a data transfer in the upward direction from a transmitting terminal to a base station requires a reservation packet, a data transfer in the downward direction from the base station to a receiving terminal does not require the reservation packet.

[0050] The base station provides each mobile terminal with reference timing in data packet transmission/reception operations using a pilot signal transmitted through a pilot channel in parallel with the transmission of the data packet 14. Since each mobile terminal can receive the data packet 14 and the pilot signal transmitted from the base station with the same delay time, the mobile terminal can readily accomplish synchronization acquisition, when receiving the data packet 14, by determining the timing of a receiving time slot based on the pilot signal.

[0051]FIG. 3 shows a reservation based access control in a conventional FDMA radio communication system.

[0052] As described above in connection with FIG. 2A, the reservation based access control is a control method in which a reservation packet is sent prior to the transmission of a data packet, and the data packet is transmitted after the reservation is established. For this control, the reservation channel 7 and the reply channel 8 are provided in addition to the traffic channels 9. The channels may be divided in accordance with the time division multiple access (refer to the prior art 2) other than the frequency division multiple access (refer to the prior art 1) shown in FIG. 3.

[0053] In FIG. 3, the abscissa represents the time axis 21. When a radio terminal transmits a reservation packet to a base station through the reservation channel 7, the base station schedules time slots on the traffic channels, and transmits a reply packet indicative of a reservation result to the radio terminal through the reply channel 8.

[0054] In the conventional reservation based access control, if a plurality of radio terminals transmit reservation packets onto the reservation channel 7 at a time, the reservation packets may collide with each other and collapse, as indicated by 22 a, 22 b in FIG. 3, with the result that the base station cannot receive the reservation packets. Each radio terminal determines that its reservation packet would have collided with any other reservation packet on the reservation channel if a reply packet destined thereto has not been returned in a predetermined time period after the radio terminal had sent the reservation packet. In this event, the radio terminal again transmits the reservation packet (indicated by 23 a, 23 b). Thus, the throughput in a radio communication system employing the conventional reservation based access control is limited depending on the collision of reservation packets as described above.

[0055]FIG. 4 shows an access control in a reservation based CDMA radio communication system according to the present invention.

[0056] The present invention applies CDMA packet transmission to a reservation channel to allow a plurality of radio terminals to transmit reservation packets individually at arbitrary timing.

[0057] In a reservation channel 7 illustrated in FIG. 4, the ordinate represents transmitting terminals 25. FIG. 4 represents a situation in which the transmitting terminals 25 have transmitted reservation packets partially overlapped on the time axis 21.

[0058] In the CDMA scheme, the spread-spectrum is applied by replacing each symbol (bit “1” and “0”) in transmitted data with a spreading code (orthogonal code or PN code) composed of a plurality of chips having unique patterns. For example, in a direct sequence spread-spectrum, a plurality of transmitting terminals modulate transmission data using the same PN (pseudonoise) sequence, and transmit the spread-spectrum data at the same carrier frequency. In this event, if there is a time deviation of one chip or more in transmission timing between respective symbols in data, the receiving side can individually identify each of transmitted data.

[0059] If a plurality of reservation packets are transmitted at completely the same timing, the packets will collide, whereby destinations will fail to receive the reservation packets. However, generally, such transmission of a plurality of reservation packets at completely the same transmission timing is rather a rare case. In the spread-spectrum, even if two packets are time-overlapped, the collision is avoided when these packets are deviated in timing by a time equal to or longer than one chip, as indicated by 26 a, 26 b in FIG. 4, thus eliminating the need to retransmit the reservation packets. It will be appreciated that the reservation based control scheme according to the present invention significantly improves the throughput compared with the conventional reservation based communication system.

[0060] In the present invention, each radio terminal having a request for data transmission transmits a reservation packet at arbitrary timing on the reservation channel, and sends a data packet in a time slot on a traffic channel, both specified by a reply packet received through the reply channel.

[0061] The data packet is transmitted in units of time slot in principle. When transmission data is so long that a plurality of time slots are required for the transmission, the data is divided into a plurality of data packets, and a time slot is reserved for each data packet. However, for reducing overhead due to the reservation process, a plurality of time slots may be reserved by a single reservation packet such that a base station, in response to the reservation packet, assigns a plurality of continuous or intermittent time slots to a transmitting terminal by a single reply packet or a plurality of reply packets generated for respective time slots.

[0062] While the present invention allows the mobile terminals to transmit reservation packets at arbitrary timing, the mobile terminals must transmit and receive a reply packet and a data packet in synchronism with a time slot having a previously defined constant length.

[0063] As illustrated in FIG. 4, the reply channel 8 and the respective traffic channels 9 are divided into time slots respectively having a fixed length, and a pilot signal is used to match the timing, thus facilitating fast synchronization of spreading codes between each radio terminal and a base station. More specifically, the base station spreads the pilot signal (reference signal) with a spreading code (PN sequence) having a suitable period, and continuously transmits the spread-spectrum pilot signal on a common channel (pilot channel). Each radio terminal generates a synchronization signal based on the pilot channel despread from the spread-spectrum pilot signal with a PN sequence unique to the pilot channel, and sets a time slot in synchronism with the base station on the reply channel and on each traffic channel.

[0064] It should be noted that since the pilot signal is intended for the synchronization of the spreading codes, the pilot signal may include any contents. Thus, for transmitting the pilot signal, the reply channel, for example, may be utilized instead of using the dedicated pilot channel.

[0065] FIGS. 5A-5C illustrate formats for the packets used in the mobile communication system according to the present invention.

[0066] The reservation packet, as illustrated in FIG. 5A, is composed of a preamble 31 a for synchronization acquisition; a type of reservation 432 b indicative of the type of the packet (identification code for identifying a location registration packet, a link securing packet, or a traffic channel reserving packet); a source address 33 (using a local ID if the location has been registered); a destination address 34 (using a link number if a link has been secured); a number 35 of reservation desired transmission packets (time slots); and a CRC (Cyclic Redundancy Check) code 36 a serving as an error detection code, arranged in this order from the beginning. The number 35 of transmission packets is not required in the call set up process for location registration or link securing.

[0067] The reply packet, as illustrated in FIG. 5B, is composed of a source address 34; a type of reply 32 b indicative of the type of the packet (for identifying a location registration packet, a link securing packet, an upward direction information transmitting packet or a downward direction information transmitting packet); a PN type 37 indicative of a spreading code of a traffic channel to be sued; timing information 38 indicative of assigned transmission timing (time slot); and a CRC code 36 b, arranged in this order from the beginning.

[0068] It should be noted that in the present invention, the reply packet does not require a preamble. This is because each radio terminal can acquire each reply packet by receiving the pilot signal and establishing the synchronization of each time slot on the reply channel based on the pilot signal, as described above.

[0069] The data packet for transmitting information, as illustrated in FIG. 5C, is composed of a preamble 31 b; a type of packet (for identifying a location registration packet, a link securing packet, an upward information transmitting packet, or a downward information transmitting packet) 32 c; a source address 33 (using a local ID if the location has been registered); a destination address 34 (using a link number if a link has been secured); data 39 (a PN code for the information transmitting channel or the reply channel, transmission or reception timing, and transmission information); and a CRC code 36 c, arranged in this order from the beginning.

[0070] Since the reply channel and the traffic channel for transmitting information are respectively divided into packets, it is desirable that the sizes of respective packets be unified to a fixed length even if the types of packets are different. For this purpose, dummy bits may be inserted in a front portion of each packet so as to adjust the beginning position of respective fields subsequent thereto. In the downward data packet, the preamble 31 b may be omitted as is the case of the reply packet.

[0071]FIG. 6 illustrates a schematic configuration of the base station 4.

[0072] The base station 4 comprises an antenna 41; a CDMA transceiver 50; a packet controller 90; a BSC interface 42 connected to a controller (BSC 43) intervening between the base station 4 and the mobile communication network 2.

[0073]FIG. 7 illustrates in detail the configuration of the CDMA transceiver 50 in the base station. The CDMA transceiver 50 comprises receiving radio module 52 and a transmitting radio module 53 for modulating and demodulating a baseband signal as well as for transmitting and receiving signals at radio frequencies.

[0074] Referring specifically to FIG. 7, a control packet (reply packet) signal transmitted from a base station to a radio terminal is inputted to an encoder 58 a through a reply channel signal line 45 a, and is subjected to encoding for error correction using, for example, a convolutional code or the like. The encoded reply packet signal is multiplied by an orthogonal code for the reply channel outputted from an orthogonal code generator 59 in a multiplier 56 a to generate a spread-spectrum reply packet signal which is then inputted to an adder 60.

[0075] Similarly to the reply packet signal, data packet signals outputted to a plurality of signal lines 45 b respectively corresponding to traffic channels are encoded in the encoder 58 b, and multiplied by orthogonal codes corresponding to respective traffic channels in a multiplier 56 b to generate spread-spectrum data packet signals which are then supplied to the adder 60. A pilot signal outputted to a signal line 45 c is likewise encoded in an encoder 58 c, multiplied by an orthogonal code unique to the pilot channel in a multiplier 56 c to generate a spread-spectrum pilot signal which is then supplied to the adder 60.

[0076] The output of the adder 60 is multiplied by a PN code (long code) unique to each base station outputted from a PN generator 57 a in a multiplier 56 to generate a spread-spectrum signal which is subsequently supplied to the transmitting radio module 53.

[0077] On the other hand, a received signal processed by the receiving radio module 52 is inputted to a matched filter 70 a for the reservation channel and to a plurality of matched filters 70 b-70 b′ respectively corresponding to traffic channels.

[0078] The matched filter 70 a despreads the received signal with a PN code unique to the reservation channel. The despread signal is separated into a plurality of bit data trains 89 each for a corresponding reservation packet in a packet separation circuit 80. In this case, as described later with reference to FIGS. 8 and 9, if the period of a PN sequence applied to the despreading process is selected to be equal to the number of taps of the matched filter, the outputs of the matched filter can be used as despread results without further processing, thus realizing fast synchronization. Each bit data train for a corresponding reservation packet, separated from other bit data trains in the packet separation circuit 80, is subjected to a decoding process accompanied by error correction, for example, such as Viterbi decoding or the like in a decoder 55 a, and subsequently supplied to the packet controller 90.

[0079] The matched filters 70 b-70 b′ are provided for acquiring the initial synchronization of PN sequences of received signal son the respective traffic channels. Once the synchronization is acquired, each of the PN generators 57 b, 57 b′ generates a PN sequence for each channel in synchronism with the acquired PN sequence. The received signal is multiplied by PN sequences corresponding to respective channels generated by the PN generators 57, 57 b in multipliers 56, 56′ to be despread. The despread signals are accumulated for every one symbol length in accumulators 54, 54′. The accumulated results are decoded by decoders 55, 55′ and subsequently supplied to the packet controller 90 as data packet signals for the respective traffic channels.

[0080]FIG. 8A illustrates the principle of the matched filter 70 a. The matched filter 70 is composed of a plurality of cascaded delay elements 71 each having a delay time T equal to a chip width of a PN sequence; a plurality of taps arranged on the input side of the delay element at the first stage and on the output side of the respective delay elements; and a plurality of coefficient multipliers 72, one in each tap. The matched filter 70 a is configured such that received signals inputted at every chip time propagate from one tap to the next in the delay time T.

[0081] In the matched filter 70 a for the reservation channel, the delay time of each delay element 71 is equal to the chip width of a PN sequence for the reservation channel, and the number of taps is equal to the number of chips included in one period of the PN sequence, such that a one-period portion of the PN sequence simultaneously appears at the plurality of taps at the time the top chip of an inputted signal reaches the rightmost tap. Therefore, respective chip values (“1” or “−1”) of the PN sequence a1-an for the reservation channel are previously set in the respective coefficient multipliers 72 as coefficients, and a total sum of the results of multiplications of respective tap outputs by the respective coefficients is calculated by an accumulator 73. If the accumulation result is outputted as a correlation value between the received signal and the PN sequence for the reservation channel, the synchronization is acquired at the time the correlation value changing for every chip time presents a peak value. Also, the output value of the accumulator 73 at this time indicates a demodulated value generated by despreading the received signal.

[0082] In the present invention, the number of taps of the matched filter 70 a is made equal to a spreading code length so that the output 79 a of the matched filter 70 a contains information (symbol code) of a one-bit portion of the reservation packet. Also, a short code type PN sequence having a less number of chips is applied as a spreading code for the reservation channel to reduce the number of taps required to the matched filter, thus facilitating the synchronization acquisition.

[0083]FIG. 8B illustrates an output signal of the matched filter 70 a which is generated when two reservation packets A, B are partially overlapped on the time axis.

[0084] The output signal 79 a of the matched filter 70 a includes a plurality of positive peak values (indicative of a code bit “1”) and a plurality of negative peak values (indicative of a code bit “0”) generated by the accumulator 70 a. Peak values equal to or more than a predetermined threshold are detected from the output of the matched filter 70 a and grouped into groups of signals appearing at a time interval matching with the PN sequence period from the respective start points at which the first peak values are detected (synchronization acquisition time), thereby making it possible to identify a bit data train 78 belonging to the reservation packet A and a bit data train 76 belonging to the reservation packet B.

[0085] In the illustrated example, the peak value 76-1 appearing first is defined as the start point, and signal values(“1” or “−1”) 76-2, 76-3, 76-4, . . . subsequently appearing at a time interval equal to the PN period 75 are extracted from the output of the matched filter 70 a to reproduce the bit data train 76 constituting the reservation packet A. Also, a peak value 77-1 appearing asynchronously with the bit data train 76 is defined as the start point, and signal values (“1” or “−1”) 77-2, 77-3, 77-4, . . . are extracted at a time interval equal to the PN period 75 are extracted from the output of the matched filter 70 a to reproduce a bit data train 77 which constitutes the reservation packet B. By applying a similar principle, even if three or more reservation packets are transmitted in a time-overlapped condition, bit signals for each packet can be identified as long as a phase deviation over one chip or more exists between the respective packets.

[0086]FIG. 9 illustrates an exemplary configuration of the packet separation circuit 80.

[0087] The output signal 79 a of the matched filter 70 a is inputted to an absolute value circuit (ABS) 81, the output of which is compared with a predetermined threshold outputted from a threshold circuit 82 by a comparator 83 a. When the output of the absolute value circuit 81 is larger than the threshold, the output of the comparator 82 is turned ON (“1” state) and inputted to an AND circuit 84 a. Since the AND circuit 84 a is also supplied, as other input signals, with inverted signals which are initially OFF (“0” state), the AND circuit 84 is opened by the ON output from the comparator 83 a, whereby its output signal is turned ON (“1” state). The ON output from the AND circuit 84A is inputted to AND circuits 84 b and 84 d.

[0088] The AND circuit 84 b is also supplied at the other input terminal thereof with an inverted version of an output signal from a timer 85 a. In an initial state, the output of the timer 85 a is in OFF state (“0” state), so that the output of the AND circuit 84 b is also turned ON at the time the output of the AND circuit 84 a is turned ON. The ON output of the AND circuit 84 b is inputted to a timing register 86 a as an enable signal, whereby the timing register 86 a is set at a value recorded on a counter 87 which performs a counting operation at an interval equal to the chip period of the PN code and returns to an initial value at an interval equal to the symbol length. The counter 87 outputs a value which indicates a chip position at the timing at which the synchronization is acquired, as previously described with reference to FIG. 8B.

[0089] The ON output of the AND circuit 84 b causes a timer 85 a to start for controlling the other input terminals of the AND circuits 84 b and 84 d. The timer 85 a maintains its output in ON state for a time period corresponding to one reservation packet. This permits the AND gate 85 d to remain open and the AND gate 84 b to remain close until a time set in the timer 85 a expires, thus preventing any other counted value from being set in the first timing register 86 a.

[0090] If the next peak value is outputted from the matched filter 70 a before the time set in the timer 85 a expires, the ON output from the AND circuit 84 a is inputted to an enable terminal of a second timing register 86 b through a pair of AND circuits 84 d and 84 d′ which remain open. As a result, the output value of the counter 87 is set in the second register 86 b. At this time, a timer 85 b cooperating with the second timing register 86 b is started and performs a similar operation to that of the timer 85 a to prohibit any other value from being set in the second timing register 86 until a one-packet period has elapsed and to open a pair of AND gates at the next stage so as to input the subsequently generated enable signal to a third timing register 86 c.

[0091] In this embodiment, since the packet separation circuit 80 is provided with four timing registers 86 a-86 d, the synchronization acquisition timing is stored for four reservation packets, determined by the order of generation, within a plurality of reservation packets generated in a time-overlapped condition by repeating the foregoing operations in a similar manner.

[0092] The value of the synchronization acquisition timing set in the timing register 86 a is compared with an output value of the counter 87 in a comparator 83 b. Every time the counted value is coincident with the synchronization acquisition timing value set in the timing register 86 a, the output of the comparator 83 b is turned ON.

[0093] The ON output of the comparator 83 b is inputted to an enable terminal of a data register 87 a through the AND circuit 84 c which remains open while the timer 85 a is in ON state. As a result, the data register 87 a is supplied with the output of the matched filter 80 a at the synchronization acquisition timing. The remaining timing registers 86 b-86 d also operate in a manner similar to the foregoing to store the outputs of the matched filter 70 a for respective reservation packets in data registers 87 b-87 d, respectively.

[0094] Since the data registers 87 a-87 d are supplied with data in accordance with the synchronization acquisition timing of the respective reservation packets, the contents of these data registers 87 a-87 d are transferred to output registers 88 a-88 d, respectively, in synchronism with a clock having a bit period generated by a clock generator 88, and data indicative of the contents of the respective reservation packets are transferred to the decoder 55 a illustrated in FIG. 7 from the output registers 88 a-88 d.

[0095]FIG. 10 illustrates an exemplary configuration of the packet controller 90 in the base station 4.

[0096] Received data from the reservation channel (the contents of a reservation packet) is inputted to a digital signal processor (DSP) 91, and is processed by a reservation packet processing routine 92 of the DSP 91. Subsequently, an assignment of a traffic channel and a time slot (scheduling) is performed by an upward schedule control routine 93.

[0097] A traffic channel (PN type) and a time slot (timing information) determined by the upward schedule control routine 93 is transferred to a reply packet constructing unit 97 together with a source address of a reservation packet to which a reply packet is destined. The reply packet constructing unit 97 generates a reply packet including the above information and transmits it to the reply channel signal line 45 a. In this way, the operation for transmitting an upward data packet from each mobile terminal can be controlled in accordance with the scheduling of the base station.

[0098] Received data from respective traffic channels are inputted to reception processing units 96 b, 96 b′ arranged in correspondence to the respective traffic channels through signal lines 44 b, 44 b′, and transferred to the BSC interface 42 through signal lines 46 as received data packets.

[0099] On the other hand, a downward data packets outputted from the BSC interface 42 to signal lines 47, after temporarily stored in transmission buffers 99, 99′, are transmitted under the control of a schedule executed by a downward schedule control routine 95 of the DSP 91. More specifically, in accordance with a downward schedule, a reply packet constructed by the reply packet constructing unit 97 is first sent from the reply channel, and subsequently data packets generated by the traffic packet constructing units 98 a, 98 a′ are sent in predetermined time slots on traffic channels determined by the downward schedule.

[0100] In this embodiment, for restraining mobile terminals from issuing reservation packets when the traffic channels remain busy, a busy tone value calculation routine 94 of the DSP 91 generates busy tone information in accordance with the number of reservation packets received through the reservation channel and traffic channel utilization state information known to the upward schedule control routine 93, and notifies the busy tone information to the respective mobile terminals through the reply channel 45 a.

[0101]FIG. 11 illustrates the configuration of the radio terminal 5.

[0102] The radio terminal 5 is composed of an antenna 100; a CDMA transceiver 110 connected to the antenna 100; a packet controller 130 connected to the CDMA transceiver 110; and a data processing unit connected to the packet controller 130.

[0103] The data processing unit comprises a microprocessor (MPU) 101; a memory 102 for storing data and programs; and a plurality of input/output devices connected to an internal bus through an I/O interface 103. The input/output devices may comprise, for example, a camera 104 a, a speaker 104 b, a display 104 c, a keyboard 104, and so on.

[0104]FIG. 12 illustrates in detail the configuration of the CDMA transceiver 110 in the radio terminal.

[0105] The CDMA transceiver 110 comprises a receiving radio module 112 and a transmitting radio module 113. These modules are responsible for modulation or demodulation of a baseband signal and a receiving process or a transmitting process at radio frequencies.

[0106] In a transmitter circuit, a reservation packet signal outputted to a reservation channel signal line 106 a is encoded for error correction in an encoder 120 a, and then multiplied by a unique PN sequence (short code) generated from a PN generator 121 a in a multiplier 114 a to generate a spread-spectrum reservation packet signal which is sent to the transmitting radio module 113.

[0107] On the other hand, a data packet outputted to a traffic channel signal line 106 b is encoded for error correction in an encoder 120 b, and multiplied by a PN sequence (long code) generated by a PN generator 121 b in a multiplier 114 b to generate a spread-spectrum data packet which is sent to the transmitting radio module 113. The spread-spectrum for the data packet is performed using a PN sequence specified by a base station, which is identified by a control signal outputted onto a signal line 106 c by a packet controller 130 and in synchronism with reference timing 105 c provided from a PN generator 119 in a receiver circuit.

[0108] In the receiver circuit, a received signal outputted from the receiving radio module 112 is inputted to a multiplier 114 c which multiplies the received signal by a PN code unique to the base station generated by the PN generator 119 to despread the received signal. The output of the multiplier 114 c is parallelly inputted to multipliers 114 d, 114 e and 114 f respectively for the reply channel, traffic channels and pilot channel, and multiplied by orthogonal codes unique to the respective channels generated by an orthogonal code generator 117.

[0109] On a reply channel line 105 a and a traffic channel line 105 b, output signals from the multipliers 114 d, 114 e are inputted to accumulators 115 d, 115 e, respectively, to produce accumulated values for each symbol length for despreading the output signals from the multipliers 114 d, 114 e. Output signals of the respective accumulators 115 d, 115 e are inputted to decoders 116 d, 116 e, respectively, for error correction, and then transferred to the packet controller 130 through signal lines 105 d, 105 e, respectively.

[0110] On a pilot channel line 122, a pilot signal outputted from an accumulator 115 f is inputted to a DLL (Delay Locked Loop) circuit 118 for tracking of synchronization. The PN generator 119 is forced to generate a PN sequence in synchronism with the output of the DLL circuit 118. It should be noted that the decoders 116 d, 116 e on the reply channel line 105 a and the traffic channel line 105 b are operated in synchronism with the pilot signal outputted from the accumulator 115 f.

[0111]FIG. 13 illustrates an exemplary configuration of the packet controller 130 in the radio terminal.

[0112] Received data through the reply channel appearing on the signal line 105 a is inputted to a DSP 131 and precessed by a monitoring routine 132. The contents of the reply packet is supplied to an upward schedule control routine 134 and to a downward schedule control routine 135, while a busy tone signal received through the reply channel is supplied to a busy tone calculation routine 133.

[0113] Received data through a traffic channel appearing on the signal line 105 b is received by a reception processing circuit 136 which is controlled by a control signal from the downward schedule control routine 135 and a reference timing signal 105 c, and received data in a particular time slot specified by a base station through a reply packet is outputted onto a signal line 107 as receiving information.

[0114] On the other hand, transmission data from the radio terminal, after temporarily stored in a transmission buffer 138, is fetched by a traffic packet constructing unit 139 in accordance with an instruction from the upward schedule control routine 134, and is sent onto the traffic channel signal line 106 b as a data packet.

[0115] When a reply packet is received from a base station, the upward schedule control routine 134 generates a signal 106 for specifying a traffic channel (PN sequence) to which a traffic packet is to be sent, and issues a data packet sending instruction to the traffic packet constructing unit 139 at timing of a time slot specified by the base station. The traffic packet constructing unit 139, upon receiving the data packet sending instruction from the control routine 134, reads transmission data from the transmission buffer 138, and sends the data packet illustrated in FIG. 5C onto the traffic channel signal line 106 b at predetermined output timing determined based on the reference timing signal 105 c.

[0116] The busy tone value calculation routine 133 calculates a busy tone value indicative of a traffic situation from a busy tone signal received through the reply channel, and notifies the busy tone value to the upward schedule control routine 134.

[0117] The upward schedule control routine 134 controls the generation of reservation packets in accordance with the traffic situation. For example, if the busy tone signal does not indicate to restrain data transmission with transmission data being accumulated in the transmission buffer, the reservation packet constructing unit 137 is started at arbitrary timing to transmit a reservation packet to the reservation channel signal line 106 a. Conversely, if the busy tone signal indicates to restrain data transmission, the transmission of reservation packets is restrained until the traffic situation improves.

[0118] As described above, in this embodiment, the CDMA scheme is applied to the reservation channel to reduce the possibility of retransmission of reservation packets due to collision of the reservation packets even if respective mobile terminals transmit the reservation packets at arbitrary timing. Moreover, the busy tone control is added to restrain the transmission of new packets from mobile terminals when the traffic channels or the reservation channel is in an overload condition.

[0119] The CDMA has a problem that when a plurality of packets are generated in a time-overlapped condition, the packet signals mutually affect as noise, so that if a large number of packets are simultaneously generated, the receiver side cannot identify them because all packet signals are buried in noise. As described above, in the mobile communication system of the present invention comprising a reservation channel, a reply channel and a plurality of traffic channels, the total number of reply packets and data packets can be controlled by the scheduling function of the base station, whereas the base station cannot directly control reservation packets since they are issued autonomously from respective mobile terminals.

[0120] As described above, a method which allows each radio terminal to autonomously control the transmission of a reservation packet with reference to the busy tone signal from the base station is effective in avoiding concentrated reservation packets to smoothly control the transmission in each terminal.

[0121] While the busy tone signal may be transmitted through a channel dedicated thereto, empty time zones appearing periodically on the reply channel may also be utilized.

[0122] The reply channel, as shown in FIG. 2, is divided into time slots each having a length corresponding to the length of a data packet on the traffic channel based on the pilot signal. Since the reply packet includes a smaller amount of information, its length can be made shorter than the data packet. For example, assuming that the time slot length (data packet length) is 512 bits and the reply packet length is 42 bits, 12 reply packets can be transmitted through the reply channel during one time slot period on the traffic channel, with a 8-bit empty time zone remaining at the end of the time slot. It is therefore possible to utilize the available empty time zone in the time slot to periodically transmit the busy tone signal through the reply channel.

[0123] Next, a reservation packet restraining method using the busy tone signal transmitted in an empty time zone on the reply channel will be described with reference to FIGS. 14A, 14B.

[0124] In FIG. 14B, “t−1”, “t” and “t+1” designate time slot numbers on the reply channel, and a pulse waveform represents the busy tone signal 143. The busy tone signal 143 is periodically transmitted utilizing an empty time zone left in each time slot on the reply channel.

[0125]FIG. 14A shows a relationship between a total amount of packets sent out by radio terminals in each time slot and a number T of allowed packets which can be transmitted in a time-overlapped condition. An area 148 indicates an amount of reservation packets sent in the time slot “t−1” and an area 149 indicates an amount of data packets sent in the time slot “t−1”.

[0126] In the following, the busy tone signal generated by the base station in the time slot “t−1” will be described, assuming that a number of transmitted data packets during the time slot “t” is I(t), a number of transmitted reservation packets is R(t), a number of transmission requested reservation packets is R(t)′, and a transmission probability of reservation packets is P(t). Further, R(t)′ and R(t) are defined to be numbers of reservation packets when the length of the reservation packet is normalized by the length of the data packet.

[0127] First, assume the following equation (1): $\begin{matrix} {{R(t)}^{\prime} = \frac{R\left( {t - 1} \right)}{P\left( {t - 1} \right)}} & (1) \end{matrix}$

[0128] Assuming that the number R(t)′ of transmission requested reservation packets possessed by all radio terminals in the service area of a base station in the time slot “t” is equal to a number R(t−1)′ of transmission requested reservation packets in the previous time slot “t−1”, the equation (1) is derived by substituting a number R(t−1) of reservation packets actually received by the base station as the value of R(t−1)′. To the base station, the number I(t) of data packets in the time slot “t” is known from previously received reservation packets and the result of scheduling the traffic channels for received data packets from other base stations.

[0129] Thus, the value of R(t)′ is estimated from the equation (1), and when a total amount of the number R(t)′ of transmission requested reservation packets and the number I(t) of data packets in the time slot “t” exceeds a tolerable value T as shown by the following equation (2), the transmission of reservation packets is restrained by the busy tone signal:

I(t)+R(t)′≧T  (2)

[0130] In this event, the transmission of reservation packets is controlled by the busy tone signal such that the transmission probability P(t) of reservation packets from radio terminals in the service area is restrained by a traffic amount on the traffic channels, as shown by the following equation (3), thereby making the sum of the number of reservation packets and the number of data packets substantially equal to the tolerable value T. Since the number of reservation packets actually transmitted from radio terminals is determined from the probability, it is desirable that the tolerable value T be set at a slightly lower level in order to allow for a certain margin. $\begin{matrix} {{P(t)} = \frac{\left\{ {T - {I(t)}} \right\}}{{R(t)}^{\prime}}} & (3) \end{matrix}$

[0131] On the other hand, if a total amount of packets estimated in the time slot “t” is in a relationship expressed by the following equation (4), the transmission of reservation packets is controlled by the busy tone signal such that the transmission probability P(t) follows the equation (5), thus allowing all radio terminals to freely transmit reservation packets.

I(t)+R(t)′<T  (4)

P(t)=1.0  (5)

[0132] The base station may notify respective radio terminals of information indicative of the transmission probability expressed by the equation (3) or (4) as the busy tone signal 143 in the time slot “t−1”.

[0133] As will be apparent from the foregoing description, the present invention applies CDMA to a reservation based packet access control type mobile communication system to reduce the possibility of retransmission of reservation packets due to their collision, even if each mobile terminal is allowed to transmit a reservation packet at its arbitrary timing, to improve the throughput.

[0134] According to the present invention, for example, a short spreading code is applied to a reservation packet, and the synchronization is acquired on the base station side using a matched filter, so that even if a plurality of mobile terminals transmit reservation packets asynchronously to each other, the base station can identify the respective reservation packets at a high speed. Also, a reduced local address (own address) shorter than an original address number or a link number (destination address) is used for terminal address information set to each packet, so that the transmission efficiency can be improved. Further, when each terminal is allowed to control the transmission of reservation packets in accordance with a busy tone signal from a base station, it is possible to avoid an excessive amount of reservation packets simultaneously communicated on a channel, thus ensuring a favorable communication environment.

[0135] It is to be understood that the above-described embodiments are merely illustrative of the principles of the invention and that may variations may be devised by those skilled in the art without departing from the spirit and scope of the invention. It is therefore intended that such variations be included within the scope of the claims. 

What is claimed is:
 1. A communication method in a code division multiple access (CDMA) mobile communication system wherein radio channels between a base station and a plurality of mobile stations include a plurality of traffic channels used for transmitting upward data packets directed from the mobile terminals to the base station and for transmitting downward data packets directed from the base station to the mobile terminals, a reservation channel used for transmitting reservation packets each indicative of a traffic channel assignment request from a mobile terminal to the base station, and a reply channel used for transmitting reply packets each indicative of a traffic channel through which data is to be transmitted and received from the base station to a mobile terminal, and said reservation, reply and traffic channels are assigned unique spreading codes, respectively, said method comprising the steps of: transmitting a reservation packet from a mobile terminal having a request for data transmission onto said reservation channel at arbitrary timing; transmitting a reply packet onto said reply channel from the base station to specify a traffic channel and a time slot to be used to each mobile terminal which has transmitted said reservation packet; and transmitting a data packet from each mobile terminal which has transmitted said reservation packet in the specified time slot on the traffic channel specified by said reply packet.
 2. A communication method according to claim 1, wherein: said each mobile terminal having a request for data transmission applies a spreading code unique to said reservation channel having a period shorter than a spreading code applied to the spreading of a data packet to be transmitted on said traffic channel in order to apply spread-spectrum to said each reservation packet; and said base station processes a received signal using a matched filter in which said spreading code unique to said reservation channel is set, said base station utilizing the periodicity of said spreading code to separate outputs of said matched filter into a plurality of signal trains corresponding to respective reservation packets.
 3. A communication method according to claim 1, further comprising the steps of: giving a local address shorter than unique address information assigned to each mobile terminal from said base station to said each mobile terminal; wherein said base station sets a local address in a destination address field of said reply packet and transmits said reply packet; and receiving the packet including a local address given to each mobile terminal in the destination address field by said each mobile terminal.
 4. A communication method according to claim 1, further comprising the steps of: giving a link number shorter than address information unique to each mobile terminal from said base station to said each mobile terminal; wherein said each mobile terminal transmits a data packet including said link number as a destination address onto a traffic channel.
 5. A communication method according to claim 1, further comprising the steps of: periodically transmitting from said base station busy tone information indicative of a traffic situation in its service area through said reply control channel or through a channel dedicated to the busy tone information; wherein each mobile terminal having a request for data transmission controls the transmission of a reservation packet based on said busy tone information.
 6. A communication method according to claim 5, wherein: said base station estimates, based on a number of reservation packets received during a previous constant period, a number of reservation packets to be generated in the next constant period, and generates said busy tone information based on said estimated value and a number of packets scheduled to be transmitted during the next constant period.
 7. A communication method according to claim 1, wherein: said each mobile terminal requests the transmission of a plurality of data packets through a single reservation packet.
 8. A communication method according to claim 3, wherein: said base station uses a control packet for giving a local address to a mobile terminal to specify a reply channel through which the mobile terminal is to perform receiving operations,
 9. A communication method according to claim 1, wherein: said base station continuously transmits a pilot signal including a synchronization signal component through a pilot channel or through said reply channel such that each mobile terminal identifies a time slot on said traffic channel based on said received pilot signal.
 10. A communication method in a radio communication system wherein a base station and a plurality of radio terminals communicate through a reservation channel, a reply channel and a plurality of traffic channels formed in accordance with a code division multiple access (CDMA) scheme in radio channels, said method comprising the steps of: transmitting reservation packets onto said reservation channel from said respective radio terminals requesting data transmission asynchronously with each other; after separating a plurality of reservation packet signals having partially overlapped portions on a time axis, received through said reservation channel, corresponding to reservation packets, and performing a receiving process on the reservation packets, transmitting from said base station a reply packet for specifying a traffic channel and a time slot to be used to each radio terminal which is a source of each received reservation packet through said reply channel; and transmitting a data packet in a specified time slot on a traffic channel specified by said reply packet from said each radio terminal.
 11. A communication method according to claim 10, further comprising the steps of: after transmitting, from said base station to a radio terminal identified by a destination address in a data packet received from said traffic channel, said reply packet for specifying a traffic channel and a time slot to be used by said ratio terminal for reception through said reply channel, transmitting said received data packet in said specified time slot on said specified traffic channel; and receiving the data packet in said specified time slot on said traffic channel specified by said reply packet at the radio terminal which is the destination of said reply packet.
 12. A radio terminal apparatus for communicating with a base station through radio channels comprising: means for transmitting a reservation packet processed with a spreading code unique to a reservation channel to said base station asynchronously with said base station; means for receiving a reply packet processed with a spreading code unique to a reply channel, transmitted from said base station at predetermined time slot timing synchronized with said base station; and means for transmitting and receiving a data packet processed with a spreading code unique to a particular traffic channel at predetermined time slot timing on said particular traffic channel specified by said reply packet.
 13. A base station for communicating with a plurality of radio terminals, each having a unique address, using spread-spectrum packets through a reservation channel, a reply channel and a plurality of traffic channels, each of said channels corresponded to a unique spreading code, comprising: means for receiving reservation packet signals each for requesting a traffic channel access transmitted by the plurality of radio terminals asynchronously with each other through said reservation channel, and separating said reservation packet signals into individual reservation packets for a receiving process; means for transmitting through said reply channel a reply packet for specifying a traffic channel and a time slot to be used to a radio terminal which is a source of said reservation packet; and means for receiving a data packet transmitted from a radio terminal or for transmitting a data packet to a radio terminal in each time slot on a traffic channel.
 14. A mobile communication system comprising a base station and a plurality of mobile terminals, radio channels between the base station and the plurality of mobile terminals comprising a plurality of traffic channels used for transmitting upward data packets directed from mobile terminals to a base station and for transmitting downward data packets directed from the base station to the mobile terminals, a reservation channel used for transmitting reservation packets each indicative of a traffic channel assignment request from a mobile terminal to the base station, and a reply channel used for transmitting reply packets each indicative of a traffic channel through which data is transmitted and received from the base station to a mobile terminal, each of said reservation, reply and traffic channels being assigned a unique spreading code in accordance with a code division multiple access (CDMA) scheme, wherein: said each radio terminal comprises first means for transmitting a reservation packet onto said reservation channel at arbitrary timing when a request for data transmission is issued, and a second means for transmitting and receiving a data packet in a time slot specified by a reply packet transmitted from the base station through said reply channel; and said base station comprises first means for separating a plurality of reservation packet signals having partially overlapped portions on a time axis, received through said reservation channel, into reservation packets and performing a receiving process on said reservation packets, and fourth means for transmitting a reply packet for specifying a traffic channel and a time slot to be used, through said reply channel, to each radio terminal which is a source of each received reservation packet.
 15. A mobile communication system according to claim 14, wherein: said first means of said each mobile terminal applies a “spreading code unique to the reservation channel” having a period shorter than a spreading code applied to a data packet transmitted through the traffic channel by said second means to spread said each reservation packet; and said third means of said base station processes a received signal with a matched filter in which said spreading code unique to the reservation channel is set, and separates an output of said matched filter into a plurality of signal trains corresponding to said packets utilizing the periodicity of said spreading code.
 16. A mobile communication system according to claim 14, wherein: said base station includes means for periodically transmitting busy tone information indicative of a traffic situation in its service area through said reply control channel or through a channel dedicated to the busy tone information; and said each mobile terminal includes means for controlling the transmission of a reservation packet based on said busy tone information.
 17. A mobile control system according to claim 16, wherein: said base station includes means for estimating, based on a number of reservation packets received during a previous constant time period, a number of reservation packets to be generated during the next constant time period; and said busy tone information is generated based on said estimated value and a number of packets scheduled to be transmitted during the next constant time period. 